I heard that a person is not supposed to have in mind a certain person they would want to marry, when they daven, because it might not be the right person for them. is this true or not?
The Gemara writes that a person should not daven for a specific person to marry, because she might not be suitable, and because he might come to doubt the force of tefillah if he is not answered. A number of authorities explain the potential problems of praying for a specific person to marry in different ways.
Sources: See Sefer Chassidim 383 (the marriage will lead to a speedy divorce); Ran on Gemara (he will have to either divorce her or die); see also Novelos Chochmah, Mifalos Elokim, Peleh Yoetz (zivug); Tashbatz (2:1); Tzemach Tzeddek on Gemara.